I just tried to reproduce this problem, and what I saw is:
1) The boot block is getting wedged in an infinite loop because it's
reading bogus data for the root directory. I fixed the infinite loop.
2) The reason it's getting bogus data has to do with inconsistent
geometry translation. When the BIOS and NetBSD geometries are the same,
I am unable to reproduce the problem.
So, it looks like this is in fact your problem.
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